In direct sequence code division multiple access (DS-CDMA), the binary information sequences individually generated by the users are multiplied by a respective very long pseudo-random sequence whose elements have a period (chip time) much smaller than the bit time of the information sequences, so that the energy of the signals from each user is uniformly spread over the whole system band. The pseudo-random sequence is typical of the user and represents a code for the same. The spread spectrum signals thus obtained are added together and sent to all users. At the receiving side, the information is recovered through decorrelation operations, by taking advantage of the code sequence. This technique maintains communication secrecy.
An essential requisite of a system using this technique is power control, which is particularly critical in the mobile-to-base link, and yet is also necessary in the base-to-mobile link in order not to affect the system capacity in comparison with the other link, for a given transmission quality.
In fact, in the mobile-to-base link it is essential that the signals sent towards the base station from different mobile stations locked thereto arrive at the base station ideally with the same power, because in the absence of power control the signals from a mobile station near the base station could completely mask the signals from a more distant out mobile station. To deal with this problem (commonly called the "near-far problem" in the art) a high level of dynamics is required for power control, usually in excess of 80 dB. In the base-to-mobile link, power control is less critical, as each mobile station receives from its base station a signal which is the sum of the useful signal (the signal actually intended for the mobile station itself) and of interfering signals (those intended for all other mobile stations). Both the useful signal and the interfering signals follow the same path within the cell, and there is no such problem as explained above. However, the interference due to signals from the base stations of adjacent cells must be taken into account, and it has been found that only by applying power control also in the base-to-mobile link a carrier/interference ratio of the same order as in the reverse link can be obtained. In this way, capacity is essentially the same in both links, for a given quality.
As regards power control in the mobile-to-base link, according to the most common technique each mobile station corrects the power level sent according to an estimate of the attenuation undergone in the base-to-mobile link by a pilot signal of known power level transmitted by the base station. Since the mobile-to-base link operates in a frequency band different from that of the base-to-mobile link, the attenuation estimated as above is not the same affecting the signal in the mobile-to-base link. Therefore, the power level of each mobile station is then corrected upon command from the base station according to the power level received at the latter. This technique is described for instance in the paper "On the Capacity of a Cellular CDMA System", by K. S. Gilhousen et al., IEEE Transactions on Vehicular Technology, Vol. 40, No. 2, May 1991, pages 303-311.
The possibility of carrying out a power control in the base-to-mobile link based on the distance of each mobile station from the base station is described in the paper "Power Control in CDMA" submitted by W. C. Y Lee to the VTS Conference, St. Louis, U.S.A., May 19-22, 1991, published on pages 77-80 of the Conference Proceedings. As a result of such control, the overall power transmitted by the base station is reduced, in that the base station transmits at a higher power level to mobile stations at the boundaries of the cell and at a lower power level to mobile stations near the base station. Practical application of this type of control is extremely difficult, as it causes continuous variation of the total power transmitted by the base station in a cell, which in turn prevents control of interference in adjacent cells.
Moreover, even if such a control could be implemented, it has been recognized in the literature that its performance would not be satisfactory (see the paper "Power Control in Cellular DS CDMA Systems, submitted by R. Prasad, M. Jansen and A. Kegel to COST 231 TD(92)-92, Helsinki, Finland, Sep. 8-11, 1992) and that better results can be obtained by performing power control in the base-to-mobile link on the basis of an estimate of the carrier/interference (C/I) ratio of each mobile station. This paper states that in this way the C/I ratio of each user is minimized according to his demands, although no indication is given of how to obtain such a result.